The present invention relates to ceramic precursor tapes and their use in the manufacture of electronic components.
The preparation of dielectric ceramics is similar to the preparation of other ceramics in that a green, i.e., unfired, piece is first prepared and then fired to obtain a desired ceramic article. In the preparation of multilayer capacitors, preformed sheets, films or strips of green, dielectric material are coated with a pattern of an electrode ink, stacked, compressed and fired to provide a monolithic multilayer capacitor. For a given dielectric material, capacitance is a function of thinness, i.e., capacitance increases as the thickness of a dielectric layer decreases. Thus, it is possible to reduce capacitor size while simultaneously increasing the capacitance of the capacitor. Theoretically, there is almost no limit to the smallness of multilayer capacitors. However, practical size limitations exist due to the nature of green tapes and the problems associated with their handling.
The physical properties of green tapes become a critical factor in the preparation of thinner, higher capacitance, dielectric layers. Lack of strength makes green tapes susceptible to tearing. Lack of flexibility leads to cracking. Thus, in the preparation of multilayer capacitors and similar devices having thinner dielectric layers, it is essential to produce a thin green tape strong enough to be free-standing and capable of maintaining its physical integrity throughout the processes of ink screen printing, laminating, dicing, etc.
Tapes typically are prepared by tape-casting or knife-coating a slip onto a nonporous carrier. The slip typically contains a ceramic powder, a binder, a wetting agent, a solvent, and a plasticizer, depending on the specific application. A wide variety of materials are used as the binder. For example, Thompson, J. J., in Ceramic Bulletin, Vol. 42, pp. 480-1 (1963), discloses the use of organic binders such as methylcellulose, starch derivatives, or preferably polyvinyl alcohol. U.S. Pat. No. 2,736,080 discloses the use of cellulose or cellulose pulp in the production of dielectric sheets having an approximate thickness of 5 mils. U.S. Pat. No. 2,759,854 discloses the use of an ester gum binder in the preparation of green films having a thickness in the neighborhood of 3 mils or less. UK No. 1,493,102 discloses a process for the preparation of glass microchanneled bodies by dipping a filament into a solution of a ceramic powder and a polymeric film-former. Polyvinyl alcohol, polystyrene, ethylcellulose, cellulose nitrate, and dialkoxy polytitanates are listed as suitable film-formers. U.S. Pat. No. 3,495,996 discloses the use of natural gums, synthetic resins, cellulose resinous materials, and the like, in the preparation of encapsulated electronic devices. U.S. Pat. No.3,536,508 discloses the use of binders such as solid ethylcellulose resins, solid polymers of an acrylate or methacrylate ester of a 1-4 carbon aliphatic alcohol, polyvinyl alcohol or polyvinyl butyral as temporary binders for ceramic particles.
U.S. Pat. No. 2,966,719 discloses the preparation of a green ceramic tape having a thickness as thin as on the order of 1 mil using binders such as cellulose acetate butyrate resin compatibly plasticized with dimethyl phthalate or tricresyl phosphate, or polyacrylate esters. However, the preparation of said tape requires that the tape be cast onto a flexible support. U.S. Pat. No. 3,619,220 discloses the preparation of a fired ceramic having a thickness of approximately 1 mil using ethylcellulose as a binder. However, the product is fired on a glass plate and is not taught to be free-standing.
U.S. Pat. No. 2,486,410 discloses an early process for the preparation of flat ceramic plates using binders such as ethylcellulose. The resulting tapes are described as being "leather-hard."
U.S. Pat. No. 3,189,978 discloses a process for the preparation of multilayer circuits by first preparing a plurality of dry, thin films, each comprising finely divided ceramic particles and a heat-volatile binder therefor. Said patent discloses that a film having a thickness of about 1 mil is flexible and strippable from the carrier upon which it was cast. The binder was vinyl chloride-acetate copolymer. A metal-containing ink is applied to the previously described film. The metal ink is taught to contain binders such as methylcellulose, ethylcellulose, or nitrocellulose. The patent broadly teaches that the thickness of unfired ceramic films may be varied between about 0.5 and 20 mils.
U.S. Pat. No. 4,447,853 discloses the preparation of multilayer capacitors having individual dielectric layers of from 10 to 30 microns in thickness. The only statement in said patent regarding the binder is that "the manufacturing method of the multilayer capacitor is the same as the prior art method."
Heretofore, a flexible, strong, thin, free-standing, ceramic green tape having a thickness of less than about 10 microns (ca. 0.4 mils) has not been disclosed. Nor has it been disclosed that the use of ethylcellulose having specific properties is especially advantageous in the preparation of such a tape. Such a tape would be desirable for the preparation of improved multilayer capacitors and related devices.